1. Field of the Invention:
This invention relates to an AGC circuit used in a FM radio receiver, for example, and more particularly to an AGC circuit having an improved intermodulation disturbing property and an improved adjacent channel disturbing property.
2. Description of the Related art:
FIG. 7 of the accompanying drawings illustrates an equivalent circuit of a conventional AGC circuit used in a vehicle-mounted FM radio receiver. This AGC circuit, performing AGC by detecting the output signal level of a mixer, is disclosed in "'88 SANYO SEMICONDUCTOR DATA BOOK, CAR AUDIO BIPOLAR IC VOLUME (CQ PUBLISHING COMPANY)" at pages 124-136.
In FIG. 7, the numerals designate, respectively: 10, a AGC circuit (AGC) integrated on an IC 12, for example; 14, an antenna circuit (ANT), the output of which is supplied to a first gate of a RF amplifier 16, and to which a pin diode 18 being an attenuator (ATT) of the ANT 14 is coupled to control the gain (attenuating amount) of the ANT 14; 20, a mixer (MIX) to which antenna signals having been amplied in the RF amplifier and local oscillating signals from a local oscillator (OSC) through a buffer 24 are input to perform a frequency conversion from the RF to the IF; 26, a level detector for detecting the level of an IF signal being the output signal from the mixer 20, the detected level signal being input to AGC 10.
The AGC 10 generates an AGC signal when the signal level becomes equal to or above a predetermined level to execute gain control of the antenna circuit 14 and the RF amplifier 16.
Namely, such an AGC is necessary because the level of the signal to be input to the radio receiver greatly changes depending on the distance from the transmitting channel, propagating conditions or the like, and it must be adusted to be within the optimum range of the processing carried out in the receiver. In particular, in a vehicle-mounted receiver, since the distance from the transmitting channel changes with the passage of time, and the propagating condition is greatly different depending on the running position of the vehicle, the antenna input level would change greatly within a range of 1 .mu.V-3 V, for example. Accordingly, such an AGC circuit would be indispensable to provide a satisfactory receiving state even in case of the signal level being low, and to provide a non-jamming signal reception suppressing the stresses caused by the higher harmonic wave produced by a non-linear element such as a transistor, FET, or a variable capacitance diode at the time of receiving extremely high level signals.
The AGC signal is applied to the pin diode 18 and the second gate of the RF amplifier 16 through the 6th pin and the 13th pin of the IC 12, respectively, so as to reduce the level of the RF signal. In this manner, the AGC 10 functions to keep the output level of the mixer 20 constant by the AGC signal.
In this case, the relationship of the mixer input frequency to the AGC sensitivity will be represented as shown in FIG. 8. As shown in FIG. 8, the larger the detuning degree .DELTA.f with respect to the central frequency (.DELTA.f=0) of a desired channel, the lower the AGC sensitivity.
If no AGC is provided, a third higher harmonic wave may be caused by non-linearity of, e.g., Tr, when the input levels of ANT and MIX become larger, thereby degradating the IM (intermodulation) disturbing property. Therefore, the AGC is provided to attenuate the RF signal and to suppress the generation of these stresses, in order to improve the disturbing property.
As disturbances to be considered in the AGC circuit, an intermodulation disturbance (three signal property) and a adjacent channel disturbance (two signal property) can be listed.
The former is a disturbance generated by mixing the higher harmonic wave caused by the non-linearity of the element due to the level of the large signal input with a desired station frequency. For eliminating this intermodulation disturbance, the signal level at a frequency band remote from the desired channel frequency is attenuated by the AGC circuit. Therefore, it is preferable to set the AGC property to a broad band AGC.
On the contrary, the latter adjacent channel disturbance occurs when a strong electric field disturbance channel exists near the desired channel, and the desired channel is a weak electric field.
Namely, for generating an AGC signal from the mixer output in the above-mentioned prior art, if an adjacent channel has a large electric field intensity with respect to the desired channel, the AGC would emit the AGC signal to the adjacent channel due to the property represented in FIG. 8. At this time, the signal level of the desired station tends to be simultaneously attenuated, consequently, the mixer input becomes extremely small.
Further, when an AGC signal is generated from the mixer input by improving the conventional circuit, the AGC circuit would operate incorrectly due to a local oscillating signal. Namely, as mentioned above, a local oscillating circuit composed of the local oscillator 22 and the buffer 24 is provided at the front end of the FM receiver, and the signal level, 300 mVrms-1 Vrms, is quite higher than the mixer input level. In addition, the frequency of the local oscillating signal is so high as 50 MHz-120 MHz, near the mixer input signal band. Therefore, the local oscillating signal would tend to invade the AGC circuit through the GND line, the Vcc line, or the space propagation, generating an erroneous AGC signal due to the local oscillating signal level. Such a operation of the AGC circuit would reduce the gain of the RF amplifier significantly, thereby degradating the sensitivity.
Therefore, in case of applying AGC to a detected mixer input level, the sensitivity of the AGC must be reduced. But by this reduction, an additional problem, that of the disturbance (eliminating) property with respect to a frequency particularly having a small detuning degree .DELTA.f is degraded, would newly arise.
Conventionally, as a further improved AGC circuit, keyed AGC is well known. In this keyed AGC, the controlling level of the AGC circuit is compensated by the use of the IF section output level of the radio receiver.
Thus, in this conventional circuit, the AGC controlling amount is reduced or cut off when the output level of the IF section is low so as to prevent the sensitivity degradation.
However, in such a keyed AGC, there has occur an inconvenience that the keyed AGC would operate even in a non-signal state having no output level, thereby significantly amplifying the intermodulation disturbance.